The ability to produce amorphous metals, also called metallic glasses, from the liquid phase in significant sizes has long been pursued. However, practical production limitations imposed by the need for a combination of rapid cooling, immaculate process environments, and alloy compositions have limited the applicability of known production processes.
Perhaps the most touted quality of metallic glasses is their combination of mechanical strength, elasticity, hardness, and toughness. Crystalline metals/alloys above a very small scale have lattice defects disrupting the long-range atomic ordering. These defects are generally the initiation sites of mechanical failure. Without crystals and such crystal defects, amorphous metals tend to outperform their crystalline counterparts in strength and elasticity. In addition to their mechanical strength, the lack of grain boundaries and lattice defects makes the amorphous alloys resistant to corrosion and wear, rendering them suitable as components in harsh chemical/mechanical environments. Moreover, since amorphous alloys can maintain flow at relatively low temperatures without crystallizing, they can be molded into complicated shapes using techniques similar to thermoplastic molding.
A metallic glass is expected to have an electrical conductivity two orders of magnitude lower than the metal/alloy in its crystalline structure. As a result, efforts are being made to achieve not just the mechanical strength of metallic glass but also improved electrical conductivity. Moreover, it has been observed that metallic glasses of ferromagnetic materials can exhibit soft magnetization (i.e., almost no hysteresis in the B-H diagram as the magnetic field is cycled above and below zero). This property translates into very low losses when employed as magnetic cores in transformers or other magnetic components.
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid below its melting point without it becoming crystallized. Thermodynamically, the preferred state for most materials is a crystalline solid if the temperature is below the melting point of the particular material. The crystallization process is always initiated by one or more nucleation events in the liquid. The nucleation process is categorized as either heterogeneous or homogeneous, where heterogeneous nucleation is aided or catalyzed by a foreign element (e.g., for instance entrained impurities or the container wall), and homogeneous nucleation is induced by the base metal itself. For either category, nucleation is a random process, and the driving force increases with undercooling. Once a nucleus of sufficient size has formed, crystal growth ensues. However, if a liquid can be sufficiently supercooled, the kinetics of crystallization become prohibitively slow, and the liquid becomes frozen in an amorphous solid state without a crystalline structure. The temperature range where this occurs is called the glass transition range, and it differs from one material to the next.
Generally, in order to reach glass transition for metallic liquids, the liquid needs to be cooled sufficiently fast from the melting point down to glass transition in order to avoid nucleation and crystal formation. The necessary cooling rate depends on the material, and most efforts in the prior art are concerned with finding good glass formers—that is, alloy compositions that have inherently slow crystallization kinetics and/or a glass transition that is close to the liquidus temperature of the system.
There are several empirical rules for creating a good glass former. Among these rules is the notion that good glass formers tend to include at least three different elements and that these should differ by at least 12% in atomic radius. The stoichiometry of such glass-forming compositions also tend to lie close to deep eutectics. Such compositions tend to have a lower mobility when undercooled and, therefore, require a more modest critical cooling rate. Cooling a melt at a rate that is higher than this critical rate will bypass crystallization, and the melt will solidify as glassy. Indeed, limited to techniques known in the prior art, many metallic glasses can only be made with a thickness on the order of millimeters. Additionally, in order to achieve significant supercooling, it is generally considered necessary to operate in immaculate process environments to remove foreign substances and external nucleating agents in the melt. If such nucleating agents are present, the melt tends to undergo heterogeneous nucleation.